OCR A Level Physics A: Wave motion, polarisation and wave effects (H556)

This study guide teaches section 4.4.1 Wave motion, statements (a) to (g), of the OCR A Level Physics A (H556) specification, Version 3.0 (March 2026), for first assessment in 2017. It is full A level content in Module 4, Electrons, waves and photons. Module 4 content is assessed in Exploring physics (component 02) and in Unified physics (component 03), and all components include synoptic assessment.

Use it with the revision notes and the practice questions. The OCR A Level Physics hub and the printable checklist list every outcome. To find your weak spots first, try the free 10-minute diagnostics. Measurement skills are in Foundations of physics and Development of practical skills.

What this section covers

Part	Skill
4.4.1(a)	Progressive waves; longitudinal and transverse waves
4.4.1(b)(i)	Displacement, amplitude, wavelength, period, phase difference, frequency, speed
4.4.1(b)(ii)	Use an oscilloscope to determine frequency
4.4.1(c), (d)	f = 1/T and v = fλ
4.4.1(e)	Graphs of transverse and longitudinal waves
4.4.1(f)(i)	Reflection, refraction, polarisation and diffraction of all waves; diffraction is significant when wavelength is comparable to the gap
4.4.1(f)(ii), (iii)	Ripple tank techniques; polarisation with microwaves and light
4.4.1(g)	Intensity I = P/A; intensity ∝ (amplitude)²

Progressive waves (4.4.1(a))

A progressive wave carries energy from one place to another. The particles of the medium (or the fields, for electromagnetic waves) oscillate about fixed positions. They do not travel with the wave.

• A transverse wave has oscillations perpendicular to the way the energy moves. Examples: waves on a rope, ripples on water, electromagnetic waves.
• In a longitudinal wave the oscillations lie along the way the energy moves. This produces compressions (particles close together) and rarefactions (particles spread apart). Example: sound in air.

Quantities and equations (4.4.1(b)(i), (c), (d))

• Displacement is the distance of a point on the wave from its equilibrium position, in a stated direction.
• Amplitude is the maximum displacement.
• Wavelength λ is the separation of neighbouring points that oscillate in step, for example crest to crest.
• Period T is the time for one complete oscillation. Frequency f is the number of cycles each second, in hertz. f = 1/T.
• Wave speed v is how far the wave pattern advances each second. v = fλ.
• Phase difference compares the stage two points have reached in their cycles. Points a distance x apart along a wave have a phase difference of x/λ of a cycle, which is 360x/λ degrees or 2πx/λ radians. Points one whole wavelength apart are in phase. Points half a wavelength apart are in antiphase.

Worked example 1: ultrasound

An ultrasound transducer emits a wave of frequency 40 kHz. The speed of sound in air is 343 m s⁻¹. Find the wavelength, the period, and the phase difference between two points 3.0 mm apart along the wave.

λ = v/f = 343 / 40 000 = 8.575 × 10⁻³ m = 8.6 mm
T = 1/f = 1 / 40 000 = 2.5 × 10⁻⁵ s
x/λ = 3.0 / 8.575 = 0.350 of a cycle
phase difference = 0.350 × 360 = 126° = 2.20 rad

Using an oscilloscope to find frequency (4.4.1(b)(ii))

An oscilloscope shows how a signal’s voltage varies with time. The time base setting fixes how much time each horizontal division represents. The Y-gain sets the volts per vertical division.

1. Connect the signal source to the input and set the time base so that at least one or two complete cycles fill the screen.
2. Adjust the trigger and Y-gain until the trace is stable and fills much of the screen height.
3. Count the horizontal divisions for one cycle, or better for several cycles, and divide by the number of cycles.
4. Multiply by the time base setting to get T, then use f = 1/T.

Measuring across several cycles reduces the percentage uncertainty in T. Amplitude comes from the vertical size: half the peak-to-peak height in divisions, times the Y-gain.

Worked example 2: oscilloscope

One cycle of a trace spans 3.6 divisions and the time base is 0.50 ms per division. Find the frequency.

T = 3.6 × 0.50 × 10⁻³ = 1.8 × 10⁻³ s
f = 1/T = 1 / (1.8 × 10⁻³) = 556 Hz

Graphs of waves (4.4.1(e))

Two graphs describe the same transverse wave, and you must not mix them up.

Graph	Shows	Read from it
Displacement against distance	The whole wave at one instant, like a photograph	Wavelength, amplitude
Displacement against time	One point on the wave as time passes	Period, amplitude

For a longitudinal wave you can plot the same graphs, where displacement is the shift of a particle along the direction of travel. Take displacement to the right as positive. A compression sits where the particles on its left have moved right (positive) and those on its right have moved left (negative). So the centre of a compression is at a zero crossing where the graph slopes downwards. A rarefaction centre is at a zero crossing where the graph slopes upwards. Do not describe a longitudinal wave as having crests.

Wave effects (4.4.1(f)(i))

All waves, transverse or longitudinal, show these effects.

• Reflection: the wave bounces off a boundary. The angle of incidence equals the angle of reflection. Frequency, speed and wavelength are unchanged.
• Refraction: a wave changes direction when its speed changes as it crosses a boundary. The frequency stays the same because it is fixed by the source. Since v = fλ, a lower speed means a shorter wavelength.
• Diffraction: waves spread out when they pass through a gap or around an obstacle. The effect becomes significant when the wavelength is comparable to the gap width. With a gap much wider than λ, the wave passes through mostly straight, with only slight spreading at the edges. With a gap about the same as λ, the wave spreads into a wide arc. Wavelength, frequency and speed do not change in diffraction.
• Polarisation: see below. This effect is only possible for transverse waves.

Ripple tank techniques (4.4.1(f)(ii))

A ripple tank is a shallow tray of water. A lamp above the tank casts the wave pattern onto a screen or sheet of paper beneath, where crests and troughs show as bright and dark lines.

• A vibrating dipper or bar, driven by a motor at a known frequency, makes circular or straight waves.
• Metal or plastic barriers placed in the tank show reflection, and gaps between barriers show diffraction. Narrow the gap until it is close to the wavelength and the spreading becomes obvious.
• A flat sheet of glass or plastic placed on the bottom makes a shallow region. Waves slow down there, the wavelength shortens and the wavefronts bend, which shows refraction.
• A stroboscope flashing at the dipper frequency makes the pattern look stationary, so it can be measured. Measure the distance across several wavelengths with a ruler and divide. Correct for any magnification between the tank and the screen.

Worked example 3: ripple tank

A dipper vibrates at 12 Hz. Measured on the tank, six wavelengths span 72 mm. Find the wave speed. In the shallow region, the wavelength is 9.0 mm. Find the speed there.

λ = 72 / 6 = 12 mm = 1.2 × 10⁻² m
v = fλ = 12 × 1.2 × 10⁻² = 0.144 m s⁻¹
shallow: v = 12 × 9.0 × 10⁻³ = 0.108 m s⁻¹

The frequency is 12 Hz in both regions. The ratio of speeds is 0.108/0.144 = 0.75, the same as the ratio of wavelengths.

Worked example 4: is diffraction significant?

Microwaves of frequency 10.0 GHz travel at 3.00 × 10⁸ m s⁻¹.

λ = 3.00 × 10⁸ / 10.0 × 10⁹ = 0.0300 m = 3.0 cm

Through a gap 3.0 cm wide the waves spread widely, because the gap is comparable to λ. Through a gap 30 cm wide the spreading is slight.

Polarisation (4.4.1(f)(i), (iii))

A wave is plane polarised when its oscillations are confined to one plane, which contains the direction of travel. Polarisation is possible only for transverse waves: a longitudinal wave oscillates along its line of travel, so it has no sideways plane to select. This is why polarisation is evidence that light is transverse, and why sound cannot be polarised.

With microwaves. A transmitter sends out plane polarised microwaves. A receiver (a probe or horn connected to a meter) detects them.

1. Align the receiver with the transmitter and note the meter reading.
2. Rotate the receiver slowly about the line joining them. The reading falls to a minimum, close to zero, after a quarter turn, and recovers at a half turn.
3. Place a metal grille of parallel rods between them. The signal is strong when the rods are perpendicular to the electric field oscillations and weak when they are parallel. Rotating the grille shows the same pattern.

With light. A Polaroid filter transmits oscillations in one plane only. Look at a lamp through one filter, then place a second in front and rotate it. The brightness falls to a minimum and then recovers, and over a full turn this happens twice. With the filters at 90° (crossed) almost no light passes. Light reflected from water, glass or a wet road is partly polarised, so a Polaroid filter turned the right way reduces glare. Polaroid sunglasses use this.

Intensity (4.4.1(g))

Intensity is the power passing through unit area that faces the oncoming wave.

I = P/A        unit: W m⁻²

For a point source radiating equally in all directions, the power spreads over a sphere of area 4πr². So I = P/(4πr²) and the intensity falls to one quarter when the distance doubles.

Intensity is proportional to (amplitude)². Doubling the amplitude gives four times the intensity. This is the link between the size of an oscillation and the energy a wave carries.

Worked example 5: radio transmitter

An isotropic transmitter radiates 150 W. Find the intensity at 2.5 km, the power collected by a receiving panel of area 0.80 m² facing it, and the intensity at 7.5 km.

I = P/(4πr²) = 150 / (4π × 2500²) = 1.91 × 10⁻⁶ W m⁻²
power collected = I × A = 1.91 × 10⁻⁶ × 0.80 = 1.53 × 10⁻⁶ W
at 7.5 km (three times further): I = 1.91 × 10⁻⁶ / 9 = 2.12 × 10⁻⁷ W m⁻²

Worked example 6: amplitude

The intensity of a wave rises from 0.40 W m⁻² to 0.90 W m⁻². By what factor does the amplitude change?

intensity ∝ amplitude², so amplitude ratio = √(0.90 / 0.40) = √2.25 = 1.5

Common errors

• Reading the period from a displacement-distance graph, or the wavelength from a displacement-time graph.
• Forgetting to multiply the number of divisions by the time base, or using divisions per cycle when the trace shows several cycles.
• Saying that frequency changes when a wave is refracted. It does not.
• Saying that diffraction needs a gap “smaller than” the wavelength. The effect is significant when they are comparable.
• Suggesting that sound can be polarised, or that polarisation shows a wave is longitudinal.
• Using I ∝ A for amplitude and intensity. It is the square.

Next steps

Work through the practice questions, then condense the section with the revision notes. The diagnostics help you decide what to revise next.

Official syllabus

OCR A Level in Physics A (H556) specification, Version 3.0 (March 2026), first assessment 2017, published by OCR: section 4.4.1 Wave motion, statements (a) to (g).